Measured ice nucleating particle concentrations improve the simulation
of mid-level mixed-phase clouds over the high-latitude Southern Ocean
Abstract
Climate models exhibit major radiative biases over the Southern Ocean
owing to a poor representation of mixed-phase clouds. This study uses
the remote-sensing dataset from the Measurements of Aerosols, Radiation
and Clouds over the Southern Ocean (MARCUS) campaign to assess the
ability of the Weather Research and Forecasting (WRF) model to reproduce
frontal clouds off Antarctica. It focuses on the modeling of thin
mid-level supercooled liquid water layers which precipitate ice. The
standard version of WRF produces almost fully glaciated clouds and
cannot reproduce cloud top turbulence. Our work demonstrates the
importance of adapting the ice nucleation parameterization to the
pristine austral atmosphere to reproduce the supercooled liquid layers.
Once simulated, droplets significantly impact the cloud radiative effect
by increasing downwelling longwave fluxes and decreasing downwelling
shortwave fluxes at the surface. The net radiative effect is a warming
of snow and ice covered surfaces and a cooling of the ocean. Despite
improvements in our simulations, the local circulation related to
cloud-top radiative cooling is not properly reproduced, advocating for
the need to develop a parameterization for top-down convection to
capture the turbulence-microphysics interplay at cloud top.